This invention relates to detection of the presence of media, in particular fluids.
The detection of the presence of fluids has many applications. For example, domestic heating systems preferably have a sensor to detect whether or not liquid is present in the heat exchanger and whether or not it is flowing in order to confirm that the pump is running. Detectors are also employed in many other applications, such as engine coolant systems for vehicles and domestic appliances such as refrigerators, washing machines etc.
A fluid sensor may comprise a probe including one or more thermistors. The thermistors are preheated by applying a voltage from a heating circuit. When the probe is exposed to fluid, the fluid conducts heat away from the thermistor reducing its temperature. As the effective thermal conductivity of fluids such as air, oil, water and moving fluids such as moving water are different, the presence of each of these at the surface of the probe can be determined remotely by measuring the reduction in temperature of the thermistor. Such a flow sensor is disclosed in EP-A-1 647 813.
A thermistor may also be kept at a substantially constant predetermined target temperature. The constant predetermined target temperature may be at a set level above the surrounding ambient temperature, such as by 20 to 30° C. By measuring the power needed to keep the thermistor at the constant temperature, a rate of power dissipation into the surrounding media can be determined, and thus the presence/absence or nature of the fluid in which the thermistor is provided may be determined.
Electrical power is generally applied to a thermistor by a constant current source. When the thermistor is at the required predetermined temperature, a suitable resistance load is introduced into the circuit supplying electrical power to the thermistor so that the proportion of electrical power and thus heating of the thermistor is reduced to prevent it from heating up further, beyond the predetermined target temperature. However, the introduction of a resistance load wastes a considerable amount of power, especially if the detector is used for long periods of time or constantly.
It would be desirable to be able to reduce power consumption in a fluid detector.